On the Road

We still haven't come to terms with brake adjustment

September 11, 2011

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Today is the beginning of Brake Safety Week in North America. CVSA inspectors will be doing a lot of brake stroke inspections this week, and also working with drivers, mechanics and others to improve awareness of brake systems and regulatory compliance. Its one of the best things CVSA does.
Automatic brake adjusters are poorly understood by driver and mechanics alike. Photo by Jim Park
Automatic brake adjusters are poorly understood by driver and mechanics alike. Photo by Jim Park


Unfortunately, drivers -- and I'm sure some fleets sometimes -- see Brake Safety Week and Operation Air Brake as just another cash grab. But the fact that more than 50% of commercial motor vehicle out-of-service violations cited during roadside inspections are for brake-related violations indicates there is a major knowledge gap there somewhere.

Vehicle users either do not know what constitutes proper brake adjustment or can't identify certain brake related problems, or they just aren't doing sufficiently thorough vehicle inspections and the problems are going undetected. Either way, the industry has to pull up its socks, or the creeper cops will pull them up for us.

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Automatic Brake Adjusters

While brake violations account for most of the OOS citations written at roadside, the vast majority of those are for brakes beyond their adjustment limit. Aside from the hassle of lying under the truck and visually inspecting brake stroke -- which usually requires that the driver mark and measure applied brake stroke -- brake stroke violations should be the easiest tickets in the world to avoid.

In the days prior to ABAs, drivers routinely adjusted their brakes, yet back then, the OOS rate for brake adjustment was much higher than it is today. ABAs have no doubt contributed to the lower numbers, but as current OOS figures indicate, there's still a problem.

I think it has a lot to do with the term "automatic." I think it creates the impression that the adjuster will somehow keep brake stroke within limits under any circumstance, and that's clearly not the case.

I asked Ron Plantan, the principle engineer for the wheel-end group at Bendix Spicer Foundation Brake why, given the prevalence of ABAs today, we're still seeing a relatively high number of trucks sidelined by poor brake adjustment.

"Truck brakes are a system. You have a brake chamber, a slack adjuster, and what we call the foundation brake, which includes the drums, shoes, linings, rollers, cams, etc., [all the stuff at the end of the axle]. The problem of over-stroking can originate in any one of those," he says. "The easiest thing to do is blame the most obvious suspect, the automatic adjuster."

In the days prior to automatic brake adjusters, there was a good chance the culprit was the slack adjuster simply because it hadn't been checked recently.

"ABAs are designed to compensate for wear of the brake linings and brake drums," Plantan says. "If there's a problem somewhere else in the brake system, the ABA can compensate only to a point. An overstroking ABA is a good indicator of a brake problem that needs attention."

As an industry, we have a problem with brake adjustment -- a few of them actually.

* We have a brake component whose name suggests it doesn't require much attention;

* In bygone days, the solution to an out-of-adjustment condition was to adjust the slack adjuster and make the brake stroke measurement right. That's not the correct approach with ABAs; and

* We appear to have a demonstrated lack of awareness of the role and function of ABAs.

Never Adjust an Automatic Brake Adjuster

Measuring pushrod travel is the accepted way of gauging brake system condition at roadside, but many of today's ABAs are designed to operate very close to their legal adjustment limit. When you have problems anywhere in the foundation brake, worn bushings, or if it's not returning properly to its zero-stroke position, any of those situations can result in an overstroke condition, even though the ABA itself is functioning perfectly.

With automatic adjusters, adjusting the brake may alleviate the symptom, but it may not cure the problem.

"It's real easy for someone to say, 'aw just adjust it and let it go and save the money. I can do that in a couple of minutes.' In reality, they are just covering up a problem and actually making it worse," Plantan says. "We caution drivers and mechanics not to continue adjusting the ABA if it continuously is in an overstroke condition. The fact that it won't stay properly adjusted is an indication of a problem. If you continue to adjust the brake, not only are you covering up the problem, but you could get to a point where, if you're adjusting the ABA all the time, you'll wear out the mechanism prematurely.

ABAs, under normal circumstances, should not be manually adjusted. The adjustment mechanism is there for the initial installation. They are not meant to be adjusted regularly as the older manual slack adjusters were. Continual adjustment of an ABA will cause premature wear to the clutches and ratchets inside the device, eventually rendering it unable to hold its adjustment.

A key indicator of a brake system problem on a truck with ABAs is an overstroke condition. If it's going out of adjustment constantly, it's telling you there is a problem somewhere. It could be the ABA itself, but chances are there is excessive wear or an out-of-tolerance condition somewhere in the wheel-end portion of the brake system.

And of course, the easiest way of determining proper brake system condition is to get under the truck and inspect it regularly -- including checking pushrod travel.


For more on the perils of manually adjusting automatic brake adjusters, read this report from the National Transportation Safety Board on a fatal wreck in Glen Rock, Penn. The executive summary tells the story, but the details are worth reading and sharing with shop personnel and drivers.

Click here for more information on www.operationairbrake.com.


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Author Bio

Jim Park

Equipment Editor

Truck journalist 13 years, commercial driver 20 years. Joined us in 2007. Specializes in technical/equipment material (including Tire Report), brings real-world perspective to test drives.

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